Rechargeable batteries are well known in the prior art. Rechargeable batteries are capable of being charged prior to initial use and recharged after being discharged. Generally, rechargeable batteries are charged by a battery charger having a power supply that can provide a supply of DC current. A rechargeable battery accepts the electrical current and converts it into chemical energy. As long as the rechargeable battery is capable of converting the electrical current into chemical energy, the rechargeable battery will not significantly rise in temperature. When a rechargeable battery is at full capacity, it is incapable of converting the charge current into chemical energy and it dissipates any continuing charge current as heat. The heat generated by a rechargeable battery is an ideal parameter to sense that it has reached a fully charged state.
A typical low-cost battery charger provides a charging current that is a relatively low current to a rechargeable battery such that it can be sustained indefinitely without damaging the battery. This low current, typically between 25 milliamps and 100 milliamps, will safely charge a battery from a discharged state to a fully charged state in approximately 4 to 12 hours. This low current provided by the low cost battery charger is sometimes referred to as a trickle charge. The trickle charge current can be set to a level where the battery can safely dissipate excess current into heat without overheating the battery. Generation of excessive heat in a rechargeable battery will cause it to breakdown and reduce its useful lifetime. A disadvantage to using a low current and low cost battery charger is that it requires charging a battery for a relatively long period of time in order to reach a fully recharged state. Using certain precautions, rechargeable batteries can be charged at a faster rate using higher charging currents.
A rechargeable battery can be charged at higher rates provided that safety precautions are taken to prevent overheating of the battery thereby preventing a possible fire, injury to a user, or damage to the battery or the battery charger. Preventing injury to a user is particularly important when a charging system is utilized by children to recharge batteries that are utilized in toys. Additionally, as new fast charge technology is applied to rechargeable batteries for use within toys, safety precautions become very important as a result. A battery charger should assure that a rechargeable battery is not charged at an excessively high rate and that the charging current is removed or reduced, such as to a trickle charge rate, shortly after the battery reaches its fully charged state. The charge rate refers to the level of charge current and the time to recharge a discharged battery. A charge rate is excessive if it exceeds the rate at which a rechargeable battery can convert the charge current into chemical energy. This occurs when the charging current level is higher than the maximum charge current rated for a given battery type and capacity. For example, a typical 50 milliamp-hour Nickel-Cadmium (NiCad) battery can safely be charged up to a charging current level of 200 milliamps while a 700 milliamp-hour NiCad battery can be safely charged up to a charging current level of 2.8 amps. Typically, NiCad battery construction will allow for a battery cell to be recharged at four to ten times its hour rating of battery capacity. Battery manufacturing techniques vary from manufacturer to manufacturer as well as from cell type to cell type which dictates the maximum charge rate for each cell. If the charge rate is excessive, the battery produces heat to dissipate the energy provided by the excessive charge current level. Regardless of the charge current level, when a battery reaches its fully charged state it is no longer capable of converting the charge current into chemical energy. In this case, the battery dissipates the extra charge current as heat and the current should be removed or reduced such as to a trickle charge current in order to avoid damage, maintain battery life, and protect persons and property from harm.
There are a number of types of battery chargers available that will provide for higher rates of charging. These battery chargers are referred to as high-speed chargers or fast chargers. A number of these fast chargers attempt to automatically detect the battery capacity and set an appropriate charge current level. However, fast chargers which attempt automatic detection of battery capacity usually never charge at their fastest charge rate. Instead, because there are so many batteries of varying types from different battery manufacturers having different specifications, typically the lowest battery specification is used to avoid damage. Other fast chargers require that an operator manually select the proper charge current level for the battery that is to be charged. Typically these manually set fast charges allow a charge rate and charge time to be set at the discretion of the operator. An operator can inadvertently set the battery charging parameters to dangerous levels which could result in damage to the battery charging equipment or the operator or others nearby. Others battery chargers are dedicated to a single battery type and capacity with the battery charger designed to supply current levels required for the single battery type. These dedicated chargers typically have a charge rate set to recharge a rechargeable battery outside of an hour or more. The foregoing charge current levels may include a maximum level for a fast charge and other lower levels such as a trickle charge current level for slow charge. To avoid charging a battery after having reached its fully charged state, a number of methods may be employed to provide automatic charge shut-off.
The reader is referred now to FIG. 1 illustrating a cutaway perspective view of a prior art rechargeable battery pack 100. Rechargeable battery pack 100 includes a number of rechargeable batteries 101 coupled in series to generate increased electrical capacity over that of a single rechargeable battery. Typically battery cells are coupled in series to attain the appropriate voltage level for the application. Each rechargeable battery 100 has a positive terminal and a negative terminal. In coupling the battery in series, the positive terminal of the first battery is coupled to the negative terminal of the second battery and the positive terminal of the second battery is coupled to the negative terminal of the third battery and so on. A connecting wire 103 is coupled to the negative terminal of the first battery in the series at one end and the negative battery pack contact 105 at its other end. A connecting wire 104 is coupled to the positive terminal of the last battery in the series at one end and the positive battery pack contact 106 at its other end. In some instances, a battery pack 100 may include a thermistor 110 within the battery pack housing 102 for sensing the temperature of the batteries. The resistance value of this thermistor is representative of the heat generated during a recharging process. The battery pack 100 includes the sensor contacts 115 and 116 that connect to the thermistor 110 by connecting wires 117-118 respectively.
Prior art methods of providing automatic shut off usually evaluate the rate of change in battery voltage over time (-delta V/delta time) or by evaluating the rate of change in battery temperature over time (delta T/delta time) and compare it with battery specifications. The battery temperature in prior art battery packs 100 is measured by the included thermistor 110. The measurement of temperature provided by the thermistor 110 is signaled to a battery charger through the wires 117-118 and sensor contacts 115-116. This type of battery charger will typically include a microprocessor to evaluate the rate of change in battery voltage over time or the rate of change in battery temperature over time to provide automatic shutoff. A microprocessor is particularly useful when multiple battery specifications need to be compared in a battery charger designed to charge a wide array of battery types and capacities. However, in charging systems that rely on the microprocessor to provide automatic shutoff, it is possible for a program error, power glitch, or other malfunction to cause the microprocessor to bomb or freeze. When the microprocessor bombs or freezes, often times the battery charger continues to charge a rechargeable battery without the automatic shutoff feature provided by the microprocessor. If this were to happen to the microprocessor, an accident may occur. It is also possible in chargers designed with a microprocessor to have manual input of charge rates and times. Manually inputting charging rates and charge times can result in batteries being overcharged, resulting in battery damage, charger damage or operator injury. It is desirable to provide added safety features to a battery charging system in order to avoid injury to persons and property.
In order to provide a measure of battery temperature, thermistor 110 is usually manufactured as part of the rechargeable battery pack. Including a thermistor in the manufacture of the rechargeable battery pack adds considerable expense. Thus, it is desirable to eliminate the thermistor in the rechargeable battery pack while maintaining a means for measuring the battery temperature for the purpose of automatic charge shut-off. One prior art means of removing the thermistor from the rechargeable battery pack is provided by U.S. Pat. No. 4,616,171 entitled "Battery Charger Including Thermistor" which issued Oct. 7, 1986 to Jean Hernandez and Alain Verdier ("Hernandez"). In the Hernandez patent, a thermistor, "Ther", is provided within a battery charger. In Hernandez, the battery charger casing 121 requires a side opening 120 and the battery pack housing 202 requires a side recess or cutout to allow the thermistor to couple to the rechargeable battery pack 200 inserted into the battery charger. Hernandez requires that a contact plate 100 of the thermistor mechanically and electrically couple to the electrically conductive casing 206 of an end battery cell 204 through the side recess or cutout 212. In Hernandez, the contact plate 100 and the thermistor are supported by supporting wings 57 which are mechanically deformed to allow pivoting and proper coupling when a rechargeable battery pack is inserted. Without contact being established between the thermistor Ther and the selected battery cell of the rechargeable battery pack, the thermistor remains electrically unconnected in Hernandez. If a faulty connection between the thermistor and the rechargeable battery pack is detected, the battery charger prevents the rechargeable battery pack from being charged. The Hernandez charging circuit has only one thermistor. While it is desirable to reduce the number of thermistors, one thermistor may only sense battery temperature while ambient temperature may be ignored. Furthermore, the battery charger of Hernandez patent provides only one level of charge rate of a rechargeable battery pack. Referring to FIGS. 5-6 of Hernandez, the casing 201 of the rechargeable battery pack 200 includes contact strips 211 to which end terminals of the end battery cells 204 couple on one side while the terminals 5 and 6 of the battery charger couple on an opposite side so that end terminals of the rechargeable batteries cells in a rechargeable battery pack do not directly couple to the battery charger.
A disadvantage to the battery charger of Hernandez is that mechanical components that allow pivoting may become damaged or break and not allow proper coupling of the thermistor to a rechargeable battery pack such that the battery charger would not function. It is desirable to reduce the number of mechanical components in a battery charger at the thermistor connection to improve reliability of a battery charger.
A further disadvantage to the battery charger of Hernandez is that the addition of casing cutouts and mechanical components to include the thermistor in the battery charger increases manufacturing costs. It is desirable to provide a lower cost battery charger having a thermistor for sensing temperature to provide an automatic shutoff.
Another disadvantage to the rechargeable battery pack of Hernandez and other prior art rechargeable battery packs is that the rechargeable battery pack includes extra contact strips and or wires to couple between the battery terminals of the batteries and the battery charger terminals. It is desirable to reduce the connectors between the battery terminals and the battery charger in a rechargeable battery pack to lower the cost of manufacturing a rechargeable battery pack and to improve the charging performance provided to a rechargeable battery pack by a battery charger.
Another disadvantage to Hernandez and other charging systems is that marginal safety features are provided to assure automatic charge shutoff when a rechargeable battery pack has reached its fully charged state. It is desirable to provide a new battery charging system having redundant safety features.